The present invention generally relates to computed radiography (CR) imaging, and more particularly, to systems and methods for intensity correction of under-exposed regions within CR images using nonlinear enhancement and/or linear transformation techniques.
In computed radiography (CR), multiple cassettes containing storage phosphor plates may be positioned together with a certain amount of overlap between neighboring cassettes, so that anatomy larger than the size of a single plate can be imaged. FIG. 1 illustrates a method for acquiring CR images using multiple cassettes. An object 10 (e.g. person""s leg, as shown) to be imaged is positioned within a field of view between a x-ray source 11 and multiple image-recording cassettes 12a, 12b, and 12c. Each of the cassettes 12a-c are shorter than the length of the object 10. The image recording cassettes overlap one another with each cassette acquiring a high resolution image of a portion of the object 10 to ensure the object 10 is entirely recorded. The CR images of each cassette are then combined into a mosaic image. Advantages of this kind of image acquisition, include, for example, no artifacts caused by body motion and a perfect alignment between successive images.
There are known methods for composing a mosaic image (i.e. seamlessly combining individual CR images). For instance, one method is based on cross-correlation techniques, as disclosed in U.S. Pat. No. 6,101,238, entitled xe2x80x9cSystem for generating a compound x-ray image for diagnosis,xe2x80x9d which issued August 2000. Another method is based on reference grid lines, as disclosed in U.S. Pat. No. 5,986,279, issued Nov. 16, 1999, entitled xe2x80x9cMethod of recording and reading a radiation image of an elongate bodyxe2x80x9d. These conventional methods, however, generate artifacts in the CR mosaic image composition.
More specifically, FIG. 2 illustrates how artifacts are generated in a mosaic CR image using conventional imaging methods. In FIG. 2, two overlapping cassettes 12a and 12b are shown, which cause an under-exposed region 15 within the image captured on cassette 12b. More specifically, since the two neighboring cassettes 12a and 12b are overlapped, with cassette 12b lying behind cassette 12a when viewed from the x-ray source, an under-exposed region 15, or what is referred to herein as xe2x80x9cwhite bandxe2x80x9d region, is formed in the image captured on cassette 12b. The overlapped region in the underlying cassette 12b is less exposed thereby generating an under-exposed region 15 (or white band) in the image of the cassette 12b. The white band 15 in the image is under-exposed and not as defined as a normal exposure region 17.
In addition, due to the presence of a thick metal edge 16 of cassette 12a, the portion of the under-exposed region 15 of cassette 12b corresponding to the metal edge 16 of cassette 12a appears in the mosaic image 13 as an artifact 14, or white strip.
The presence of under-exposed regions within a CR image not only cause visual disturbances for the examining physicians, but also may hide important diagnostic information. Thus, it is very desirable to provide a method to regulate the intensities of the under-exposed image regions so that the under-exposed regions may appear equally definite and clear as other parts of the image. Such a method would ensure that important anatomies in the under-exposed regions can be brought into better visibility for diagnosis.
It is an object of this invention to provide an automatic method for correcting the image intensity of under-exposed regions (or xe2x80x9cwhite bandxe2x80x9d regions) within a CR image, so that artifacts can be removed or reduced from CR images without introducing any additional distortion to the diagnostic information.
In one aspect of the invention, a method for generating a mosaic CR image comprises acquiring a set of CR images and processing each CR image to detect an under-exposed region (or xe2x80x9cwhite bandxe2x80x9d) in the CR image. If an under-exposed region is detected, the CR image is separated into an under-exposed region and a normal exposure region. The image intensity of the under-exposed region is then adjusted to be substantially similar, or as close as possible, to the image intensity of the normal exposure region of the CR image using linear transformation techniques and/or nonlinear enhancement. The intensity corrected CR images are then combined in a mosaic image.
In another aspect of the invention, a method for adjusting the intensity of a computed radiography (CR) image comprises inputting a CR image and separating the CR image into a normal exposure region and an under-exposed region. The under-exposed region of the CR image is then enhanced. Preferably, the process of enhancing the under-exposed region comprises applying a non-linear transformation to the under-exposed region to increase the dynamic range of intensity variations of the under-exposed region. A set of intensity correction parameters is then determined using the enhanced under-exposed region and the normal exposure region. Preferably, the intensity correction parameters are determined by performing a linear regression on samples of equal size of the enhanced under-exposed region and the normal exposure region. The image intensity of the enhanced under-exposed region is then adjusted using the intensity correction parameters. Preferably, the image intensity adjustment process comprises applying a linear transformation to the enhanced under-exposed region using the determined intensity correction parameters.
In another aspect of the invention, a method for adjusting the image intensity of a computed radiography (CR) image comprises a process for automatically determining whether to enhance the under-exposed region, which prevents over-enhancement of the under-exposed region. The method comprises inputting a CR image and separating the CR image into a normal exposure region and an under-exposed region. The image intensity of the under-exposed region is enhanced, preferably using a non-linear transformation to increase the dynamic range of intensity variations of the under-exposed region. A first set of intensity correction parameters are determined using the enhanced under-exposed region and the normal exposure region, preferably by performing a linear regression of the enhanced under-exposed region and equal size sample of the normal exposure region. A second set of intensity correction parameters is further determined using the under-exposed region and the normal exposure region, preferably by performing a linear regression of equal size samples of the under-exposed region and of the normal exposure region. Then, the first and second set of intensity correction parameters are evaluated to select the set of intensity correction parameters that would provide optimal intensity correction. Preferably, the evaluation process comprises determining which set of parameters provide a minimum residual error, and then selecting the set of parameters for intensity correction which provides the minimal residual error. If the first set of intensity correction parameters is selected, the image intensity of the under-exposed region is adjusted preferably applying a linear transformation to the enhanced under-exposed region using the first set of intensity correction parameters. If the second set of intensity correction parameters is selected, the image intensity of the under-exposed region is adjusted preferably by applying a linear transformation to the under-exposed region using the second set of intensity correction parameters.
These and aspects, objects, features and advantages of the present invention will be described or become apparent from the following detailed description of the preferred embodiments, which is to be read in connection with the accompanying drawings.